1. Field of the Invention
The present invention relates to a collating device which feeds sheets of paper one after another in a printing machine, book-binding machine and the like, more particularly to a suction type sheet feeder which feeds a sheet continuously one after another from the topmost of a stack of papers on a paper table in said printing machine, book-binding machine and the like by sucking a sheet of paper using a suction head.
2. Description of the Prior Art
The sheet feeder of this type in collating device of prior art is constructed so that the topmost sheet of a stack of papers on a paper table is sucked up by a suction head and then the sheet of paper sucked thereby is fed forward by getting nipped between a pair of feed rollers, for example.
In this type of collating device, the top level of the stack of paper must be maintained always at the same height by gradually raising the paper table during the feeding of paper in order to assure the sucking up of the topmost sheet from the paper stack by the suction head. For this purpose, in the ordinary collating device generally the paper table is allowed to drive upwards and downwards by a motor and a sensor is provided to the device to monitor the topmost level of the paper stack so that the drive of said motor is controlled by a control unit based on the output of this sensor to maintain the topmost level of the paper stack at the predetermined position.
However, in case of the prior collating device having a multiple paper tables, each paper table must be provided with each sensor, motor and the like, resulting in increase of cost for the device.
To solve this problem, a collating device provided with a mechanism having springs biasing the paper table upwards, a stopper contacting with the topmost sheet of the paper stack as well as a latch and a ratchet claw and the like inhibiting sliding of said paper table to maintain the topmost level of the paper stack at the predetermined position has been proposed (U.S. Pat. No. 4,930,763).
However, the device disclosed in the prior art has a disadvantage that it can not be operated in high speed because the sliding of paper table is controlled through a ring mechanism which is not suitable for such high speed operation and because the ratchet claw can not follow such high speed operation.
Another device provided with a mechanism to maintain the topmost level of the paper stack at the predetermined position by a detection bar contacting on the topmost sheet of the paper stack, a cam, an one-way clutch and the like has been also proposed as a paper feeding device without using a sensor and a motor for raising the level of the paper table U.S. Pat. No. 4,480,826). However, such device disclosed in this patent is not sufficient in effect to reduce the cost of device.
While, in the book-binding operation any papers having a special quality such as coated papers, art papers and thick papers particularly with a bigger sheet sizes and weights (gsm) must be fed forward accurately a paper one after another, therefore the suction type collating device is suitable for this type of use.
However, the suction type collating device used for the book-binding operation has a narrower space between the paper tables allowing to load only a limited volume of papers on one table because it is generally consisted with multi-layer paper tables. Moreover, the bigger the size of paper (gsm), the fewer the number of sheets to be loaded. Therefore, in case bigger size (gsm) of sheets is fed, because feeding is completed within a short time, an operator must add new sheets continuously to the empty table.
In the prior art ordinary collating device, the device itself is programmed to stop its operation to prevent any missing page when the paper table is emptied. Therefore, even though only one table is emptied, the whole machine has to be stopped during which new sheets have to be placed on the paper table, resulting in a longer loading work than the actual operation of the device.